1. Field of the Invention
The present invention relates to a power circuit breaker and a power resistor suitable for absorbing a surge generated by power equipments such as a voltage transformer and a circuit breaker.
2. Description of the Related Art
A closing resistor is generally connected to a power circuit breaker parallelly to a breaking connection point to absorb a surge generated during a switching operation and to increase a breaking capacity. As a resistor used for the above purpose, a carbon grain dispersion ceramic resistor described in Published Unexamined Japanese Patent Application No. 58-139401 is conventionally used. The resistor is obtained by dispersing a conductive carbon powder in an insulating aluminum oxide crystal and sintering them by a clay. The resistor has a resistivity of 100 to 2,500.OMEGA..multidot.cm. The resistivity to the resistor can be advantageously changed by controlling the content of the carbon powder. However, since the resistor has low denseness, i.e., a porosity of 10 to 30%, the following problems are posed.
That is, since a heat capacity per unit volume is small, i.e., about 2 J/cm.sup.3 .multidot.deg, the temperature of the resistor is remarkably increased in accordance with heat generation caused by surge absorption. In addition, since a discharge is caused between carbon grains during absorption of a switching surge, or the resistor has a negative temperature coefficient of resistance, the resistor is easily punched through and broken, and an energy breakdown is decreased. In addition, when the resistor is exposed at a high temperature, carbon grains for controlling the resistance are oxidized. For this reason, the resistance is largely changed. Therefore, in the circuit breaker using a carbon grain dispersion ceramic resistor, a space for storing the resistor is increased, and a breaking capacity must be suppressed to be small to secure the reliability of the circuit breaker.
In recent years, in accordance with an increase in capacity of a circuit breaker caused by the technical development, a high-performance closing resistor for absorbing a switching surge is strongly demanded. In order to cope with the above demand, a zinc oxide-aluminum oxide powder resistor is disclosed in Published Unexamined Japanese Patent Application No. 61-281501, and a zinc oxide-magnesium oxide powder resistor is disclosed in Published Unexamined Japanese Patent Application No. 63-55904. In these patent applications, the following advantages are described. That is, since each of these resistors has a relatively high surge breakdown and a positive temperature coefficient of resistance, the resistor has excellent characteristics, i.e., the resistor is not easily over run. However, each of the resistors is not easily formed by a highly dense sintered body, and the production stability and the stability against a change in atmosphere are not satisfied. In addition, a heat capacity per unit volume cannot be increased. As a result, in the circuit breaker using each of these resistors, a large space is required for storing the resistor, and the breaking capacity must be suppressed to be small to secure the reliability of the circuit breaker.
In Solid-State Electronics Pergamon Press 6, 111 (1963), U.S. Pat No. 2,892,988, and U.S. Pat. No. 2,933,586, zinc oxide resistors are disclosed. In these publications, the resistivity of each of these zinc oxide resistors can be controlled within a wide range by changing contents of additives such nickel oxide (NiO) and titanium oxide (TiO.sub.2) to zinc oxide contained in a ceramic. In addition, a temperature coefficient of resistance can be changed within a range from a negative value to a positive value. However, the application and performance of the resistors which are used as power resistors are not disclosed, and the application of the resistors to a circuit breaker as a closing resistor is not disclosed.